Radiation absorbing materials (RAMs) are used in a variety of applications where it is desirable to absorb, rather than reflect, electromagnetic (EM) radiation. For example, RAMs are sometimes used in coatings for cables, antennas, or other devices to shield these devices from noise which would otherwise result from the reflection of EM radiation. Another particularly advantageous application for RAM coatings is on vehicles such as airplanes to make them less susceptible to detection by radar.
The absorption properties of RAM coatings are typically the result of a ferromagnetic material included therein. More particularly, two widely used ferromagnetic materials in RAM applications are carbonyl iron and ferrous silicide. Although both materials have been supplied in fine spherical powders capable of being compounded with elastomers for application, have similar densities, and are approximately equivalent in their energy absorbing capabilities, ferrous silicide has greater corrosion resistant properties and is more thermally stable. In particular, carbonyl iron is subject to oxidation (i.e., rusting), which may not only cause magnetic degradation but also an undesirable discoloration of the coating.
One example of a ferrous silicide RAM coating is disclosed in U.S. Pat. No. 5,866,273 to Wiggins et al. This patent is directed to a method for making an iron-silicon compound powder that includes blending magnetic materials such as carbonyl iron, iron cobalt, and/or nickel and very pure silicon powders with an activator, such as a halide salt, and then heating the mixture between 1350° F. and 1600° F. in an inert atmosphere. The result is then ground until it passes through a 200 mesh screen. The powder so formed is then heated in air to form a thin protective shell about each particle of the powder. Thereafter, the powder can be combined with a suitable binder to form a RAM coating. Each of the resulting particles in the powder has a generally spherical shape.
Unfortunately, methods such as the one described above for forming ferrous silicide compounds suitable for high temperature and/or highly corrosive environments have heretofore been very energy intensive. Such methods are also typically subject to low yields. As such, the production of ferrous silicide using such methods is, generally speaking, relatively costly. In addition, coatings produced using spherical particles may be relatively heavy. Further, because of the phenomena of skin depth, only a small portion of surface area is active in attenuating EM radiation in such coatings due to the spherical nature of the ferrous silicide particles.